1. Field of the Invention
This invention relates to seismic sources and particularly to vibrators having pre-loaded assemblies.
2. Discussion of the Related Art
Conventional seismic vibrators consist of a dual-ended linear actuator disposed above a ground contacting base plate by a support frame called a stilt structure. The stilt structure typically consists of 4 I-beam legs, each having one end rigidly connected to the base plate. The opposite ends of each leg are terminated in horizontal flanges welded thereto. A cross member, often called a spider, of I-beam steel is fastened to the top of each leg by a plurality of bolts. At a predetermined distance below the top cross member is a lower cross member welded to each leg. The linear actuator is constrained between the upper and lower cross members.
After numerous hours of operation the stilt structure described above often suffers structural failures. Most often, structural failure occurs at or near the welded connections and is caused by dynamic external loads generated by the linear actuator. The reciprocating motion of the linear actuator induces bending moments in the upper and lower cross members which in turn are partially passed to the legs.
It was discovered that structural failures could be reduced by preloading the stilt structure. A conventional preload technique involves placing spacers or shims between the upper cross member and the linear actuator piston, thus creating a gap between the connection of the upper cross member to each leg. Closing the gap in the connection by tightening the bolts, results in preloading the linear actuator piston in compression. The upper and lower cross members essentially trap the linear actuator therebetween.
A major disadvantage to the above preloading technique is that the upper cross member must be physically removed in order to change the number of shims and thus changing the amount of preload on the stilt structure.
A second disadvantage in the above technique occurs when the operating range of the system is changed, so must the amount of preload on the structure. The number of shims required to achieve the new preload can be empirically determined; however, the mechanics involved in testing is cumbersome.
Another disadvantage is in setting the proper gap between each leg and the cross member. The incorrect number or size of shims will result in an under-or over-preloaded stilt structure and a reduction in the operating life.